Photochemical reactions porphyrins

Several photoreactions of porphyrin compounds proceed by triplet states.478 Chlorophyl is, of course, involved in some important photochemical reactions, and is known to form triplet states rather efficiently. However, it is not as yet certain how or whether its triplet state is involved in photosynthesis.479... 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

Several investigations of photochemical reactions of high-spin Fe porphyrins have been reported, and the subject has been reviewed see Photochemistry of Transition Metal Complexes). When bound to anions such as CH or OH, Fe porphyrins are photochemically reduced to Fe , with production of a radical (Ch or In the case of a cofacial Fe -Zrf diporphyrin, however, the photochemical reaction produced PFe JCl... 

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

As a possible in-vitro model for one-electron transfer in photosynthesis, the photochemical reaction between hemin and Chla in pyridine solution has been studied, and it has been shown that the relatively slow reduction and oxidation of iron porphyrins can be accelerated by the presence of Chla by an order of magnitude and that light further increases the rate or reaction. Chla probably forms a rather stable complex with hemin, as is shown by fluorescence quenching experiments [Brody (17)]. 

A rather important aspect that should be considered is that interfacial quenching of dyes does not necessarily imply an electron-transfer step. Indeed, many photochemical reactions involving anthracene occur via energy transfer rather than ET [128]. A way to discern between both kinds of mechanisms is via monitoring the accumulation of photoproducts at the interface. For instance, heterogeneous quenching of water-soluble porphyrins by TCNQ at the water-toluene interface showed a clear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the excited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenching rate constant of the order of 92 M-1 cm s 1. 

For the insertion of the iron ion into the porphyrin a variety of general procedures have been described and reviewed. In most cases, these methods lead to the formation of Fe " complexes, which are then used to prepare Fe Fe , Fe, and Fe porphyrins. The most commonly employed methods for synthesizing Fe porphyrins are described below. The preparation of the Fe and Fe complexes from the iron (III) porphyrins by chemical or electrochemical means and the oxidized iron porphyrins (Fe TT-cation radicals, Fe, Fe TT-cation radicals, and Fe ) by chemically or electrochemically oxidizing the iron(III) porphyrins is described in more detail in the sections on the corresponding iron porphyrins below. Whereas Fe porphyrins can be photochemically reduced to Fe porphyrins, only a few examples of photooxidations of the iron center are known, which include laser photolysis of the co-condensation products of PFe at 15K to produce PFe = O. Typical photochemical reactions of iron and other metalloporphyrins have been summarized by Suslick and Watson. ... 

Postexposure decontaminant Materials with increased catalytic capability to make indigenous reagent decontamination (e.g., oxidative-reduction or photochemical reactions using mixed oxides, porphyrins, poly-oxometalates (POMs), or synthetic enzymes)... 

Photosensitizers, such as porphyrins, which upon excitation with UV-visible light initiate photochemical reactions which are cytotoxic to the cells of the tissue, for example, a cancer which concentrates the photosensitizer relative to surrounding tissues (Spikes, 1989). 

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